Zoom lens and imaging apparatus

Zoom lens and imaging apparatus

A zoom lens includes: a first lens group having a positive power; a second lens group having a negative power; a third lens group having a positive power; and a fourth lens group having a negative power, in this order from an object side. All of the groups move along an optical axis such that the distance between the first group and the second group gradually becomes greater, the distance between the second group and the third group gradually becomes smaller, and the distance between the third group and the fourth group gradually becomes greater, when changing magnification from a wide angle to a telephoto end. The zoom lens satisfies the conditional formulae: −2.0<fw/f2<−0.8; and −1.0<fw/f4<−0.2, wherein fw is the focal length of the entire system at the wide angle end, f2 is the focal length of the second group, and f4 is the focal length of the fourth group.Related Terms:ImagingOpticConditionalOpticalZoom Lens

This application is a continuation of PCT/JP2012/003971 filed on Jun. 19, 2012, which claims foreign priority to Japanese Application No. 2011-138434 filed on Jun. 22, 2011. The entire contents of each of the above applications are hereby incorporated by reference.

TECHNICAL FIELD

The present invention is related to a zoom lens and an imaging apparatus. Particularly, the present invention is related to a zoom lens a zoom lens having a comparatively short total length, a high angle of view, and a high variable magnification ratio, and to an imaging apparatus equipped with such a zoom lens.

BACKGROUND ART

Conventionally, zoom lenses having a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a fourth lens group having a negative refractive power, in this order from the object side, are known as disclosed in Japanese Unexamined Patent Publication No. 4(1992)-296809, U.S. Pat. No. 6,449,433, and U.S. Pat. No. 7,423,813. Such a configuration is advantageous in shortening the total length of a zoom lens, by arranging two lens groups of the telephoto type.

DISCLOSURE OF THE INVENTION

However, the zoom lenses disclosed in Japanese Unexamined Patent Publication No. 4(1992)-296809, U.S. Pat. No. 6,449,433, and U.S. Pat. No. 7,423,813 have long total lengths, particularly at the telephoto end. In addition, the field of view at the wide angle end is insufficient, particularly in the zoom lens disclosed in Japanese Unexamined Patent Publication No. 4(1992)-296809. Further, the variable magnification ratio is insufficient in the zoom lens disclosed in U.S. Pat. No. 6,449,433. Still further, the field of view at the wide angle end and the variable magnification ratio are both insufficient in the zoom lens disclosed in U.S. Pat. No. 7,423,813.

The present invention has been developed in view of the foregoing circumstances. It is an object of the present invention to provide a zoom lens having a short total length, a sufficiently wide angle of view, and a high variable magnification ratio.

A zoom lens of the present invention practically comprises:

a first lens group having a positive refractive power;

a second lens group having a negative refractive power;

a third lens group having a positive refractive power; and

a fourth lens group having a negative refractive power, provided in this order from an object side;

all of the lens groups moving along an optical axis such that the distance between the first lens group and the second lens group gradually becomes greater, the distance between the second lens group and the third lens group gradually becomes smaller, and the distance between the third lens group and the fourth lens group gradually becomes greater when changing magnification from a wide angle end to a telephoto end; and

the zoom lens satisfies the following conditional formulae:

−2.0<fw/f2<−0.8 (1)

−1.0<fw/f4<−0.2 (2)

wherein fw is the focal length of the entire system at the wide angle end, f2 is the focal length of the second lens group, and f4 is the focal length of the fourth lens group.

Here, the expression “practically comprises a first lens group . . . a second lens group . . . a third lens group . . . and a fourth lens group” means that the zoom lens may also include lenses that practically have any power, optical elements other than lenses such as aperture stops and cover glass, and mechanical components such as lens flanges, a lens barrel, an imaging element, a blur correcting mechanism, etc. The same applies to the expression “the fourth lens group practically comprises a lens 41 . . . a lens 42 . . . and a lens 43” to be described later.

Note that more desirable ranges within the ranges defined in Conditional formulae (1) and (2) are:

−1.05<fw/f2<−0.85 (1)′

−0.8<fw/f4<−0.5 (2)′.

Note that it is preferable for the zoom lens of the present invention to satisfy the following conditional formulae:

−2.0<fw/f2<−0.8 (1)

−1.0−fw/f4<−0.2 (2)

wherein fw is the focal length of the entire system at the wide angle end, f2 is the focal length of the second lens group, and f4 is the focal length of the fourth lens group.

Note that more desirable ranges within the conditions defined by Conditional Formulae (1) and (2) are:

−1.05<fw/f2<−0.85 (1)′

−0.8<fw/f4<−0.5 (2)′.

In addition, it is preferable for the zoom lens of the present invention to satisfy the following conditional formula:

0.6<fw/f3<1.5 (3)

wherein fw is the focal length of the entire system at the wide angle end and f3 is the focal length of the third lens group.

Note that a more desirable range within the conditions defined by Conditional Formula (3) is:

0.6<fw/f3<1.0 (3)′.

Further, it is preferable for the zoom lens of the present invention to satisfy both of the following conditional formulae:

0.10<fw/f1<0.18 (4)

0.10<fw/f3<0.80 (3)″

wherein fw is the focal length of the entire system at the wide angle end, f1 is the focal length of the first lens group, and f3 is the focal length of the third lens group.

In the zoom lens of the present invention, it is desirable for the fourth lens group to practically comprise a lens 41 having a positive refractive power, a lens 42 having a negative refractive power, and a lens 43 having a positive refractive power, provided in this order from the object side.

In addition, it is preferable for the lens 42 and the lens 43 to be cemented together to form a cemented lens in the zoom lens of the present invention.

In the zoom lens of the present invention, it is preferable for the lens 42 to be formed by a material having a refractive index higher than the refractive indices of the materials of the lens 41 and the lens 43.

Meanwhile, an imaging apparatus comprises a zoom lens of the present invention described above.

The zoom lens according to the present invention practically comprises: the first lens group having a positive refractive power; the second lens group having a negative refractive power; the third lens group having a positive refractive power; and the fourth lens group having a negative refractive power, provided in this order from an object side. That is, two telephoto type lens groups are arranged, and therefore the total length can be shortened.

In addition, in the zoom lens according to the present invention, all of the lens groups move along an optical axis such that the distance between the first lens group and the second lens group gradually becomes greater, the distance between the second lens group and the third lens group gradually becomes smaller, and the distance between the third lens group and the fourth lens group gradually becomes greater when changing magnification from a wide angle end to a telephoto end. Therefore, correction of aberrations and amounts of movement by the lens groups can be appropriately balanced. Thereby, a wide angle of view and a high variable magnification ratio can be obtained.

Further, the zoom lens of the present invention satisfies both of Conditional Formulae (1) and (2), and therefore the following advantageous effects can be obtained. That is, Conditional Formula (1) determines the power distribution of the second lens group with respect to the entire system. If the value of fw/f2 is less than or equal to the lower limit defined in Conditional Formula (1), the refractive power of the second lens group will become excessively great, and it will become difficult to favorably correct various aberrations. Inversely, if the value of fw/f2 is greater than or equal to the upper value defined in Conditional Formula (1), the refractive power of the second lens group will become excessively small, and it will become difficult to obtain a high variable magnification ratio while maintaining a short total length. Meanwhile, Conditional Formula (2) determines the power distribution of the fourth lens group with respect to the entire system. If the value of fw/f4 is less than or equal to the lower limit defined in Conditional Formula (2), the refractive power of the fourth lens group will become excessively great, and distortion will increase from an intermediate focal length to the telephoto end. Inversely, if the value of fw/f4 is greater than or equal to the upper limit defined in Conditional formula (2), the refractive power of the fourth lens group will become excessively small, and it will become difficult to obtain a high variable magnification ratio while maintaining a short total length. The above shortcomings can be prevented in the case that Conditional Formulae (1) and (2) are satisfied.

The above advantageous effects will become more prominent in the case that Conditional Formulae (1)′ and (2)′ are satisfied within the ranges defined in Conditional Formulae (1) and (2).

In the zoom lens of the present invention, the following advantageous effects can be obtained particularly in the case that Conditional Formula (3) is satisfied. That is, Conditional Formula (3) determines the power distribution of the third lens group with respect to the entire system. If the value of fw/f3 is less than or equal to the lower limit defined in Conditional Formula (3), the refractive power of the third lens group will become excessively small, and it will become difficult to obtain a high variable magnification ratio while maintaining a short total length. Inversely, if the value of fw/f3 is greater than or equal to the upper value defined in Conditional Formula (3), the refractive power of the third lens group will become excessively great, and it will become difficult to favorably correct various aberrations. The above shortcomings can be prevented in the case that Conditional Formula (3) is satisfied.

The above advantageous effects will become more prominent in the case that Conditional Formula (3)′ is satisfied within the range defined in Conditional Formula (3).

In addition, the following advantageous effects can be obtained in the case that the zoom lens of the present invention satisfies both Conditional Formulae (4) and (3)″. That is, Conditional Formula (4) determines the power distribution of the first lens group with respect to the entire system. If the value of fw/f1 is less than or equal to the lower limit defined in Conditional Formula (4), the refractive power of the first lens group becomes excessively small, and it will become difficult to obtain a high variable magnification ratio while maintaining a short total length. Inversely, if the value of fw/f1 is greater than or equal to the upper limit defined in Conditional Formula (4), the refractive power of the first lens group will become excessively great, and it will become difficult to favorably correct various aberrations. The above shortcomings can be prevented in the case that Conditional Formula (4) is satisfied. The advantageous effects obtained by satisfying Conditional Formula (3)″ are basically the same as those obtained by satisfying Conditional Formulae (3) and (3)′ that similarly determine the range of the value of fw/f3. However, the advantageous effects become more prominent.

In addition, in the case that the fourth lens group practically comprises the lens 41 having a positive refractive power, the lens 42 having a negative refractive power, and the lens 43 having a positive refractive power, provided in this order from the object side, in the zoom lens of the present invention, the refractive power of the fourth lens group can be increased. Therefore, lateral chromatic aberrations and astigmatic aberrations can be favorably corrected while suppressing the total length of the zoom lens.

In addition, in the case that the lens 42 and the lens 43 are cemented together to constitute a cemented lens in the zoom lens of the present invention, peripheral rays causing total reflection can be prevented, while the refractive power of each lens can be increased, which is desirable.

In the zoom lens of the present invention, the angle of view can be widened in a simple manner in the case that the lens 42 is formed by a material having a higher refractive index than the refractive indices of the materials of the lens 41 and the lens 43.

Meanwhile, the imaging apparatus according to the present invention is equipped with the zoom lens of the present invention that exhibits the advantageous effects described above. Therefore, the lens portion of the imaging apparatus of the present invention can be miniaturized, and imaging with a wide angle of view and a high variable magnification ratio is enabled.

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